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Stock-Based Compensation Plans - Additional Information (Detail) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock option compensation $ 4.9 $ 3.0
Stock Options Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number exercisable and fully vested 39,225  
Fully vested stock options with performance conditions attached 6,130  
Stock option compensation $ 4.9 3.0
Total intrinsic value exercised 2.7 2.6
Total compensation cost related to non-vested stock options not yet recognized $ 10.4  
Expected compensation cost recognized over the weighted-average period 1 year 11 months 4 days  
Stock Equivalent Units [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number exercisable and fully vested 38,588  
Fully vested stock options with performance conditions attached 35,820  
Total intrinsic value exercised $ 6.8 2.4
Expected compensation cost recognized over the weighted-average period 2 years 25 days  
Compensation cost $ 14.0 $ 7.9